The present invention relates to a low resistivity material with improved wear performance for electrical current transfer and methods for preparing same.
In a particular non-limiting aspect, the invention relates to a copper-graphite composite material prepared by a powder metallurgy (P/M) route which shows improved electrical conductivity compared with conventional copper-graphite composite materials, while maintaining higher density than other similarly prepared materials. It also relates to devices and systems including such composites.
Carbon composite materials for use in applications such as brushes and contact materials in light rail systems are known. The preparation of these materials may be via P/M techniques. However, currently available materials tend to exhibit either low conductivity or cause excessive wear of counterpart components.
The present invention seeks to provide materials and methods of preparing same which are directed to ameliorating these difficulties significantly.
According to one aspect of the present invention, there is provided a copper-graphite composite material having an IACS value of at least about 40% which has been formed by mixing, compacting and sintering mixtures of copper powder having a purity of about 99.9% and graphite powder, comprising a copper network matrix having a plurality of pores therethrough, at least some of the pores containing graphite to provide a microstructure of graphite islands in a copper network matrix.
The copper-graphite composite material more preferably has an IACS value of at least 45%, and a density of at least about 6.0 g/cm3.
Preferably, the composite materials have a density in the range from about 6.3 to 7.6 g/cm3.
The following explanation of the way in which the invention provides improved performance is offered as a likely mechanism. The invention is not dependent on, nor is it limited by the explanation.
The composite materials according to the invention advantageously exhibit a self lubricating function resulting from the formation of a transfer graphite layer onto the surface of a counterpart component. The self-lubricating function of the copper-graphite composite material effectively protects the counterpart, and thus extends the lifetime of the counterpart. This may advantageously be effective in protecting and extending the lifetime of, for example, railway electrical power transmission systems. More particularly, it is estimated that the lifetime in such an application may be extended by as much as three times relative to currently used materials.
Thus the invention provides in one aspect a material which can be mounted on a pantograph for a railway train such as a pole shoe which includes a copper-graphite composite as hereinafter described as an electrical contact for receiving power from overhead power lines. It also includes power transmission systems using such a composite.
In a preferred embodiment the IACS value of the composite material is at least 60%. As will be understood by a person skilled in the art, the IACS percentage is the standard conductivity (resistivity) used to judge a material""s property of conduction based on the International Annealed Copper Standards (IACS).
According to the invention the materials may be prepared by mixing and compacting copper and graphite powders under certain conditions, and then sintering the compacted materials. The various steps of the process may suitably be carried out under non-oxidising conditions, such as under a reducing atmosphere.
According to a further aspect of the invention there is provided a method of preparing a copper-graphite composite material comprising the steps of:
purifying copper powder by annealing copper powder in a reducing atmosphere and cleaning it:
mixing the purified copper powder and a graphite powder;
compacting the mixed powder to produce a compact, and
sintering the compact at elevated temperature for a time sufficient to form the copper-graphite composite material. The copper powder may suitably have a varying particle size of no greater than about 10 xcexcm, and the graphite powder may suitably have a particle size of no greater than about 5 xcexcm.
The conditions may include compacting the well mixed powders using a pressure in the range from about 500 to about 1600 Mpa. They may also include sintering the compacted powder in the form of compacts at a temperature in the range of 960xc2x0 C. to 1100xc2x0 C. for a predetermined period under an atmosphere of H2 and N2.
Alternatively the process may include any other process of heating and pressing such as, for example hot isostatic pressing (hipping), isolated hot pressing (IHP) or vacuum sintering.
The compaction of the copper and graphite powders following the mixing step is preferably performed by either two-directional compacting or dynamic compacting. When two-directional compacting is employed, a compressing pressure from about 500 to about 1600 MPa is applied preferably for a period of about 5-10 Minutes. The alternative to this is dynamic compacting. When dynamic compacting is employed, the shock frequency is preferably in the range from about 150 to 250 Hz. Such a shock frequency will achieve a similar result to the application of a constant pressure as described above for the two-directional compacting method.
The copper powder used is advantageously of commercial grade purity or better, and is preferably of about 99.9% purity. The varied particle size of the copper powder facilitates the optimisation of the xe2x80x9cparticle size effectxe2x80x9d on mixing of the copper and graphite powders. For example, copper powder may be used at sizes of 10 micrometers (about 600 mesh) and 40, 150, 200 and 400 mesh. Preferably the particle size of the copper powder ranges between about 5 micrometers and about 150 mesh.
The copper powder is advantageously such that oxides and thinly oxidised films are not present on the particle surfaces. As such, in a preferred embodiment, the copper powder, prior to mixing with the graphite powder, is cleaned and annealed in a controlled atmosphere which is reducing, such as a mixture of hydrogen and nitrogen. Other suitable reducing atmospheres may include carbon monoxide, hydrogen, water reformed natural gas, reducing endothermic or exothermic natural gas mixtures and/or mixtures of these with less reactive gases such as nitrogen. Preferably, this is conducted at a temperature from about 600xc2x0 C. to about 850xc2x0 C. It will be readily understood by those skilled in the art that the temperature for cleaning and annealing will depend substantially on the particle size of the copper powder.
The copper powder may also have been treated to remove unwanted impurities. A magnetic separation step may be used for this purpose. Alternatively or additionally, lighter non-magnetic materials may be removed by processes such as electrostatic or centrifugal separation.
In one form of the invention the graphite powder preferably has a particle size of no greater than about 5 xcexcm and preferably has a particle size in the range from about 1 xcexcm to about 2 xcexcm. In a preferred embodiment the graphite powder is electro-grade quality.
As is the case in known P/M processes, other metallurgical powders may be included as additives. These may include, for example, Zn, MoS2 and Si. (Note: the Si additive may be in the form of a silicate.)
As described above, the mixing of the copper and graphite powders is performed under conditions to prevent oxidation of the copper powder. Preferably, the powder mixing is performed at a relatively slow speed, such as about 150 rpm in a conventional mill.
As discussed above, the compacting of the mixed powder is advantageously performed by a two-directional compacting method or a dynamic compacting method. The upper compression pressure of about 1600 Mpa, which may be used in accordance with the present invention is substantially higher than that conventionally used in P/M techniques. This is generally about 690 Mpa. It is worth noting that the pressure here is defined as load/cross sectional area of the compacting die.
The sintering temperature of the sintering step may be in the range from about 960xc2x0 C. to about 1100xc2x0 C. The holding time in the furnace will depend on the furnace facilities as would be readily understood by those skilled in the art. The reducing atmosphere used in the sintering step preferably consists of 10% H2 and 90% N2 and provides an exothermic atmosphere in the furnace.
It will be understood that the above process is provided for exemplification only as a preferred method of forming the composite materials of the invention. Other methods may also be employed provided that these produce a composite material having the advantageous characteristics as described herein.